Using an STM8L162M8 AES hardware accelerator with … · April 2012 Doc ID 022369 Rev 3 1/22 AN3992 Application note Using an STM8L162M8 AES hardware accelerator with a CR95HF to

April 2012 Doc ID 022369 Rev 3 1/22

AN3992Application note

Using an STM8L162M8 AES hardware accelerator with a CR95HFto encrypt contactless tag data memory

IntroductionThis application note describes STM8L162M8 demonstration firmware which reads and writes encrypted data into an LRxk contactless tag. The MCU encrypts data using its embedded AES hardware and sends it to a contactless tag through the CR95HF transceiver.

The data stored into the contactless tag can be read by anyone but decrypted only by the encryption or decryption key owner.

Figure 1. Data encryption diagram

MS19972V1

ISO15693 RF STM8L162M8

Plain data Encrypted data

CR95HFLRxk

contactless tag

ISO/IEC 15693 RFtransactionSPI or UART bus

AES hardware

RF communication

www.st.com

Contents AN3992

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Contents

1 Acronyms and notational conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.1 List of terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Notational conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.1 Binary number representation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.2 Hexadecimal number representation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.3 Decimal number representation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1 AES cryptography overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 CR95HF overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3 STM8L162M8 overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 Firmware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1 AES hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 AES encryption mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3 Key derivation mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.4 Key derivation and decryption mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Application setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1.1 STM8L162M8 microcontroller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1.2 STM8L1528_EVAL evaluation board . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.2 CR95HF plug board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.3 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.3.1 ST Visual Develop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.3.2 Cosmic compiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.3.3 HyperTerminal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.4 Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.5 Pinout description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.5.1 Communication with CR95HF I/Os . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.6 Contactless tag layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.7 Using the software implementation of AES chaining modes . . . . . . . . . . 16

4.7.1 HyperTerminal welcome screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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4.7.2 Contactless tag memory initialization screen . . . . . . . . . . . . . . . . . . . . . 17

4.7.3 Reading contactless tag memory screen . . . . . . . . . . . . . . . . . . . . . . . . 18

4.7.4 Encrypting contactless tag memory screen . . . . . . . . . . . . . . . . . . . . . . 18

4.7.5 Decrypting contactless tag memory screen . . . . . . . . . . . . . . . . . . . . . . 19

5 Additional recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5.1 Firmware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5.2 Direct memory access (DMA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5.3 Encryption and decryption keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5.4 Block padding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

List of figures AN3992

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List of figures

Figure 1. Data encryption diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Figure 2. AES hardware accelerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Figure 3. AES hardware accelerator: encryption mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Figure 4. AES hardware accelerator: key derivation mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Figure 5. AES hardware accelerator: key derivation and decryption mode . . . . . . . . . . . . . . . . . . . . 11Figure 6. STM8L1528_EVAL board (Rev. A) connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Figure 7. PLUG-CR95HF-B Board I/Os . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Figure 8. Workspace organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Figure 9. Application flow chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Figure 10. HyperTerminal welcome screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Figure 11. Contactless tag memory initialization screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Figure 12. Reading contactless tag memory screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Figure 13. Encrypting contactless tag memory screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Figure 14. Decrypting contactless tag memory screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

AN3992 Acronyms and notational conventions

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1 Acronyms and notational conventions

1.1 List of terms

1.2 Notational conventionsThe following conventions and notations apply in this document unless otherwise stated.

1.2.1 Binary number representation

Binary numbers are represented by strings of digits 0 and 1, with the Most Significant Bit (MSB) on the left, the Least Significant Bit (LSB) on the right, and “0b” added at the beginning.

For example: 0b11110101

1.2.2 Hexadecimal number representation

Hexadecimal numbers are represented by numbers 0 to 9, characters A - F, and “0x” added at the beginning. The Most Significant Byte (MSB) is shown on the left and the Least Significant Byte (LSB) on the right.

For example: 0xF5

Table 1. List of terms

Acronyms Definitions

ADC Analog to Digital Converter

CISC Complex Instruction Set Computer

DAC Digital to Analog Converter

EEPROM Electrically Erasable Programmable Read-Only Memory

IC Integrated Circuit

IEC International Electrotechnical Commission

ISO International Organization for Standardization

LED Light Emitting Diode

LCD Liquid Crystal Display

FIPS Federal Information Processing Standard

MIPS Million Instructions Per Second

NFC Near Field Communication

RF Radio Frequency

RFID Radio Frequency Identification

SPI Serial Peripheral Interface

USART Universal Synchronous/Asynchronous Receiver/Transmitter

Acronyms and notational conventions AN3992

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1.2.3 Decimal number representation

Decimal numbers are represented as is, without any trailing character.

For example: 245

AN3992 Overview

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2 Overview

2.1 AES cryptography overviewThe purpose of cryptography is to protect sensitive data to avoid it from being read by unauthorized persons. There are many algorithms that implement cryptography. These techniques can be split into:

● Asymmetric cryptography algorithms: These algorithms use a key to encrypt and another key to decrypt messages. RSA and DSA are examples of this type of algorithm.

● Symmetric cryptography algorithms: These algorithms use the same key to encrypt and decrypt messages. Advanced Encryption Standard (AES), Data Encryption Standard (DES) are examples of this type of algorithm.

The Advanced Encryption Standard (AES) specifies a FIPS-approved cryptography algorithm that can be used to protect electronic data. AES exists in three versions: 128-bit, 192-bit and 256-bit.

2.2 CR95HF overviewThe CR95HF device is an RF transceiver IC for contactless application (ISO/IEC 15693, ISO/IEC 14443-3 and ISO/IEC 18092). It manages the RF communication with RFID or NFC contactless tags. It includes frame coding, RF modulation and contactless tag response decoding.

The CR95HF is a slave device. A host (such as an MCU) is required to control it.

2.3 STM8L162M8 overviewHigh-density STM8L162M8 microcontrollers have an embedded AES 128-bit hardware accelerator to off-load the CPU from encryption or decryption tasks. This AES peripheral is a fully compliant implementation of the AES standard as defined by the FIPS publication (FIPS PUB 197, 2001 November 26).

This application note applies to STM8L162M8 high-density devices with built-in AES peripheral. The software supplied with this application note provides an implementation of some commonly used AES chaining modes (ECB, CBC, CFB, OFB and CTR).

For more detailed information, you should refer to the AES section of the STM8L15x and STM8L16x microcontroller family reference manual (RM0031).

Firmware description AN3992

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3 Firmware description

3.1 AES hardwareThe Advanced Encryption Standard (AES) hardware accelerator can be used to encrypt (encipher) and decrypt (decipher) 128-bit blocks using a 128-bit key length.

Figure 2. AES hardware accelerator

The AES hardware accelerator provides four modes of operation:

1. Encryption mode

2. Decryption mode (using decryption key)

3. Key derivation mode

4. Key derivation and decryption mode (using encryption key)

MS19976V1

CR95HF and STM8L

AES hardware accelerator

128-bit cipher text

128-bit plain text

♫σCΣm2DM5kìL"ß▀⌠

Key

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3.2 AES encryption modeIn this mode, the AES accelerator performs the encryption of a 128-bit plain text using the provided 128-bit key to compute the cipher text. In the example below, the plain text “CR95HF and STM8L” is encrypted using the “Ultralow Power” key.

The cipher text, computed by the AES hardware accelerator, is:

● in ASCII format: ♫σCΣm2DM5kìL″ß▀⌠● in hex format: 0E E5 43 E4 6D 32 44 4D 35 6B 8D 05 4C 22 DF F4

(according to “code page 437").

See http://en.wikipedia.org/wiki/Code_page_437

Figure 3. AES hardware accelerator: encryption mode

MS19975V1

CR95HF and STM8L

AES hardware acceleratorEncryption mode

128-bit cipher text

128-bit plain text

♫σCΣm2DM5kìL"ß▀⌠

Encryption Key:"ultra-low power.”

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3.3 Key derivation modeAES is a symmetric cryptography algorithm. The AES hardware accelerator can compute a derived key from an encryption key that is named “Decryption key”. The decryption key is pre-computed to speed-up the decryption phase.

In this mode, the AES takes the encryption key as an input and provides the decryption key as an output. This mode takes 320 clock cycles.

Figure 4. AES hardware accelerator: key derivation mode

In the example above, the decryption key, computed from the “Ultralow Power” encryption key, is “91 DF 30 53 7A E9 49 19 92 FF 8D F8 C4 9B B7 4D" (hex format).

The algorithm given in Figure 5 provides the steps needed to use the AES hardware.

MS19718V1

AES hardware accelerator:key derivation mode

Encryption Key="ultra-low power.”

Decryption Key="91 DF 30 53 7A E9 49 1992 FF 8D F8 C4 9B B7 4D”

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3.4 Key derivation and decryption modeWhen this mode is selected, the AES hardware accelerator performs the decryption of a 128-bit cipher text using the provided 128-bit encryption key to compute the plain text. In the example below, the cipher text is “22 F6 21 81 F8 AF C0 FE 03 36 B8 95 72 CB D1 A8" and the encryption key is "ultra-low power.". The plain text, computed by the AES hardware accelerator, is "STM32L and STM8L".

Figure 5. AES hardware accelerator: key derivation and decryption mode

0E E5 43 E4 6D 32 44 4D35 6B 8D 05 4C 22 DF F4

Decryption Key="91 DF 30 53 7A E9 49 1992 FF 8D F8 C4 9B B7 4D”

AES hardware acceleratordecryption mode

128-bit cipher text

CR95HF and STM8L

128-bit cipher text

MS19974V1

Application setup AN3992

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4 Application setup

4.1 Hardware

4.1.1 STM8L162M8 microcontroller

High-density STM8L162M8 Ultralow power devices feature an enhanced STM8 CPU core providing increased processing power (up to 16 MIPS at 16 MHz) while maintaining the advantages of a CISC architecture with improved code density, a 24-bit linear addressing space and an optimized architecture for low power operations.

All high-density STM8L162M8 microcontroller features include data EEPROM and low power low-voltage single-supply program Flash memory.

The devices incorporate an extensive range of enhanced I/Os and peripherals, a 12-bit ADC, two DACs, two comparators, a real-time clock, an AES, 8x40 or 4x44-segment LCD, four 16-bit timers, one 8-bit timer, as well as standard communication interfaces such as two SPIs, an I2C interface, and three USARTs. The modular design of the peripheral set allows the same peripherals to be found in different ST microcontroller families including 32-bit families.

The STM8L162M8 can be used with the STM8L1528_EVAL board.

4.1.2 STM8L1528_EVAL evaluation board

Figure 6. STM8L1528_EVAL board (Rev. A) connectors

The STM8L1528-EVAL evaluation board is designed as a complete demonstration and development platform for the STM8 core based STM8L152M8T6 microcontroller with an I2C interface, 2 SPI channels, 3 USART channels, a 12-bit ADC, two 12-bit DACs, an LCD driver, an internal SRAM, data EEPROM and Flash program memory as well as SWIM debugging support.

The full range of hardware features on the board is provided to help you evaluate all the MCU peripherals (motor control, USART, microphone, audio DAC, LCD, IR LED, IrDA, SPI Flash, MicroSD card, temperature sensor, EEPROM… etc.) and develop your own applications. Extension headers make it possible to easily connect a daughter board or wrapping board for your specific applications.

In the STM8L1528-EVAL board REV A, the MCU is replaced by

STM8L162M8T6 device

USB connector used to supply the board and program the STM8L

RS232 connector for communication with

HyperTerminal

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An ST-LINK V2 is integrated on the board as an embedded in-circuit debugger and programmer for the STM8 MCU.

Important note

The STM8L1528_EVAL is delivered with an STM8L152M8T6 device and the firmware designed for STM8L162M8T6. Both chips are pin-to-pin compatible but only the STM8L162 includes the AES hardware.

4.2 CR95HF plug boardThe PLUG-CR95HF-B is a board which includes a CR95HF device and a matched antenna. A host configured as a master can communicate with CR95HF through the SPI bus.

The PLUG-CR95HF-B is powered through the VPS pin and no external power supply is required. It includes a CR95HF contactless transceiver, a 47 x 34 mm 13.56 MHz inductive etched antenna and its associated tuning components.

Figure 7. PLUG-CR95HF-B Board I/Os

CR95HF IRQ_OUTNSS

MOSIVPS & VPS_TX

CR95HF IRQ_INMISOSCKGND

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4.3 Software

4.3.1 ST Visual Develop

ST Visual Develop (STVD) provides an easy-to-use, efficient environment for start-to-finish control of application development (from building and debugging the application code to programming the microcontroller).

STVD is available on STM web site http://www.st.com/internet/evalboard/product/210567.jsp

4.3.2 Cosmic compiler

Cosmic is the compiler toolchain used by ST Visual Develop. There is a 1 year free license limited to 32 Kbytes of code and data which requires registration.

For further information about the license, refer to the Cosmic Software website.

4.3.3 HyperTerminal

HyperTerminal is a program available on Windows OS that you can use to connect to other computers, Telnet sites, bulletin board systems (BBSs), online services, and host computers, using either your modem or a null modem cable.

4.4 Project The project was built from MCD standard library which is included in the STM8L15x_StPeriph_Driver folder.

There are three steps to open the project:

1. Launch ST Visual Develop.

2. In the “File” menu, click on “Open WorkSpace”.

3. Locate the project folder and select the CR95HF_STM8L.stw file.

The project folder has been organized as the project and follows the layer organization of the libraries.

Figure 8. Workspace organization

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4.5 Pinout descriptionTable 2 describes the I/Os used and their configurations for the two boards.

4.5.1 Communication with CR95HF I/Os

Note: In low Power state, PC2 is set with Interrupt. All pins are named from STM8L point of view.

4.6 Contactless tag layoutIn this demonstration application, the plain text and the encrypted text are stored into the memory of the contactless tag. The locations of these fields are defined in the application. The sizes of plain text and cipher text are 128 bits.

Table 2. Communication with CR95HF I/Os

STM8L_EVAL

board pin

SPI

NameDirection Configuration

DDR (1)

1. Direction Data Register

CR1 (2)

2. Control Register 1

CR2 (3)

3. Control Register 2

PB7 MISO Input Floating No interrupt

PB6 MOSI Output Push-Pull 10 MHz

PB5 SCK Output Push-Pull 10 MHz

PB4 NSS Output Open drain 2 MHz

PC2 IRQ_in Input Pull-up No interrupt

PC3 IRQ_out Output Open drain 2 MHz

Table 3. Field layout

Block Byte 0 Byte 1 Byte 2 Byte 3

0

Plain text1

2

3

4

Cipher text5

6

7

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4.7 Using the software implementation of AES chaining modesThis firmware example both encrypts and decrypts the memory contents of a contactless tag.

The plain text is read from the contactless tag and encrypted by using the AES hardware included in the STM8L162M8 device. The cipher text is sent to the CR95HF through the SPI bus and transmitted to the contactless tag.

The encryption and decryption keys are stored in the EEPROM data of the STM8L162M8 device.

Figure 9 presents the main functions.

Figure 9. Application flow chart

MS19973V1

Start

END

Display on hyper terminal (1)

Initialize Contactless tag Display on hyper terminal (2)

Display on hyper terminal (3)

Is a tag present?No

Yes

Read 128 bits from tag

Display on hyper terminal (4)Write the cypher text

Read the cypher text

Display on hyper terminal (5)Decrypt

Encrypt

Is a tag present?No

Yes

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Note: 1 See Chapter 4.7.12 See Chapter 4.7.23 See Chapter 4.7.34 See Chapter 4.7.45 See Chapter 4.7.5

4.7.1 HyperTerminal welcome screen

Once the STM8L_EVAL board is powered up, the HyperTerminal screen displays the following text.

Figure 10. HyperTerminal welcome screen

4.7.2 Contactless tag memory initialization screen

The user shall place an ISO/IEC contactless tag close to the plug board antenna. When the CR895HF device detects the contactless tag, the first memory rows will be programmed with the following ASCII text (128 bits): "CR95HF and STM8L".

When the contactless tag memory is correctly initialized, the HyperTerminal screen displays the following text.

Figure 11. Contactless tag memory initialization screen

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4.7.3 Reading contactless tag memory screen

The next step of the example is to read the plain text from the contactless tag. It shall be "CR95HF and STM8L".

Figure 12. Reading contactless tag memory screen

4.7.4 Encrypting contactless tag memory screen

The plain text is encrypted by using the cipher key defined, written into the STM8L162M8 device.

Figure 13. Encrypting contactless tag memory screen

The cipher text is written into the contactless tag memory after the plain text (refer to Chapter 4.6).

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4.7.5 Decrypting contactless tag memory screen

The cipher text is read from the contactless tag and decoded by using the decryption key.

Figure 14. Decrypting contactless tag memory screen

Additional recommendations AN3992

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5 Additional recommendations

5.1 FirmwareThe firmware implementation of the AES chaining modes provided with this application note is intended to be used as a starting-point. This firmware may be tailored and simplified to cover only the required features. For further information, refer to the AES section in the RM0031 microcontroller reference manual.

5.2 Direct memory access (DMA)The Direct Memory Access (DMA) can be used to handle input and output phases for better performance, and to free the CPU for other tasks.

5.3 Encryption and decryption keysThe encryption key can be stored and protected in the EEPROM data memory. Refer to the “Flash program memory and data EEPROM” section in the microcontroller reference manual RM0031.

5.4 Block paddingAES-128 is a 128-bit block cryptography which means that AES encrypts a 128-bit plain text providing a 128-bit cipher text. The text to be encrypted and decrypted can be of any length and must be padded to get an exact multiple of a block size (128 bits) before being processed (encrypted or decrypted).

In the following example of plain text, "The CR95HF is RF transceiver for ISO/IEC 14443, 15693 and 18092 protocols" is split into 5 blocks. The last one, since its size is 9 bytes, is padded by seven “0”.

Padding messages with “00” are not recommended. Padding must comply with specifications such as ANSI X.923, ISO/IEC 10126 or PKCS7.

AN3992 Revision history

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6 Revision history

Table 4. Document revision history

Date Revision Changes

19-Dec-2011 1 Initial release.

07-Feb-2012 2 Updated Figure 7: PLUG-CR95HF-B Board I/Os on page 13.

02-Apr-2012 3Replaced ‘LRIxk’ and ‘LRxk’ by ‘LRxk’ in the Introduction ( text and Figure 1: Data encryption diagram).

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